Optical locking based on optical resonators with high quality factors

ABSTRACT

Techniques and devices for providing optical locking of optical resonators and lasers.

PRIORITY CLAIMS AND RELATED PATENT APPLICATIONS

This application is a divisional application (and claims the benefit ofpriority under 35 USC §121) of U.S. application Ser. No. 12/381,519entitled “Optical Locking Based on Optical Resonators with High QualityFactors” and filed Mar. 11, 2009, which claims priority to U.S.Provisional Application 61/035,608 entitled “Tunable Narrow-LinewidthInjection-Locked Semiconductor Lasers with High-Q Whispering-GalleryMode Resonators” and filed on Mar. 11, 2008, U.S. ProvisionalApplication No. 61/053,411 entitled “Very Precise Optical Lock ForBalanced WGMR Based Filter” and filed on May 15, 2008, and U.S.Provisional Application No. 61/058,487 entitled “All Optical Lock ForPhotonic Applications” and filed on Jun. 3, 2008. The entire disclosuresof the above patent applications are incorporated by reference as partof the disclosure of this document.

BACKGROUND

This document relates to devices based on optical resonators.

Optical resonators can be configured to exhibit high resonator qualityfactors for various applications, such as optical frequency referencesand optical filtering devices. A whispering gallery mode (WGM)resonator, for example, has a structure that confines light in awhispering gallery mode that is totally reflected within a closedcircular optical path. Light in WGM resonators cannot exit theresonators by optical transmission and thus can be used to produceoptical resonators with high optical quality factors that may bedifficult to achieve with Febry-Perot resonators. Light in a WGMresonator “leaks” out of the exterior surface of the closed circularoptical path of a WGM resonator via the evanescence field of the WGmode.

SUMMARY

This document describes implementations of techniques and devices forproviding optical locking of optical resonators and lasers.

In one aspect, this document provides a device that includes a laserthat produces a laser, an optical interferometer, and an opticalresonator coupled to the optical interferometer. The laser produces alaser output beam at a laser frequency. The optical interferometer islocated in an optical path of the laser output beam and includes a firstoptical path that receives a first portion of the laser output beam, asecond optical path that receives a second portion of the laser outputbeam, and an optical combiner where the first and second optical pathsmeet each other and terminate. The optical combiner transmits a part oflight from the first optical path and reflects a part of light from thesecond optical path to produce a first combined optical output. Theoptical combiner also transmits a part of the light from the secondoptical path and reflects a part of the light from the first opticalpath to produce a second combined optical output. The optical resonatoris optically coupled in the first optical path to filter light in thefirst optical path. This device includes a detection module that detectsthe first and the second combined optical outputs to produce an errorsignal representing a frequency difference between the laser frequencyand a resonance of the optical resonator, and a feedback controlmechanism that receives the error signal and tunes, one of (1) the laserand (2) the optical resonator, in response to the frequency differenceof the error signal to lock the laser and the optical resonator withrespect to each other.

In another aspect, a method for locking a laser and an optical resonatorto each other is described to include operating a laser to produce alaser output beam at a laser frequency without modulating the laserbeam; directing laser light of the laser output beam into an opticalinterferometer which includes a first optical path and a second opticalpath that intersect to produce optical interference between light in thefirst and second optical paths; optically coupling an optical resonatorin the first optical path to filter light in the first optical path;using two optical outputs of the optical interferometer to produce anerror signal representing a frequency difference between the laserfrequency of the laser and a resonance of the optical resonator; andtuning one of (1) the laser and (2) the optical resonator, in responseto the frequency difference of the error signal, to lock the laser andthe optical resonator with respect to each other.

In another aspect, a device is provided to stabilize a resonance of anoptical resonator with respect to a laser frequency from a laser. Thisdevice includes a laser that produces a laser output beam at a lasercarrier frequency; an optical resonator placed in an optical path of thelaser output beam to receive light of the laser output beam; and anoptical coupler that couples the optical resonator to the optical pathto receive the light of the laser output beam and to produce an opticaloutput. The optical coupler is structured and positioned relative to theoptical resonator to provide, at an optical filter mode frequencydifferent from the laser carrier frequency, optical coupling that is notunder a critical coupling condition under which light coupled into theoptical resonator is completely trapped inside the optical resonator.The device includes an optical modulator, in the optical path of thelaser output beam between the laser and the optical resonator, tomodulate the laser output beam to produce a modulation sideband at whichoptical coupling of the light in the modulation sideband by the opticalcoupler into the optical resonator is near the critical couplingcondition to thermally stabilize the resonator by optical absorption oflight in the modulation sideband. The modulation sideband is differentin frequency from the optical filter mode frequency.

In another aspect, a method for operating an optical resonator filter isprovided to include directing to an optical resonator a resonatorcontrolling laser beam at a laser carrier frequency that is modulated tocarry a modulation sideband; operating an optical coupler to couplelight at the modulation sideband into the optical resonator near acritical coupling condition under which light coupled into the opticalresonator is completely trapped inside the optical resonator tothermally stabilize the resonator by optical absorption of light in themodulation sideband; and directing an input optical signal, while theresonator is receiving and is thermally stabilized by the resonatorcontrolling laser beam, through the optical resonator to perform opticalfiltering of the input optical signal by a resonance of the opticalresonator at an optical filter mode frequency that is different from themodulation sideband and the laser carrier frequency of the resonatorcontrolling laser beam.

In yet another aspect, a device for locking a laser to an opticalresonator is provided to include a distributed feedback (DFB) laser thatis tunable in response to a control signal and produces a laser beam ata laser frequency; an optical resonator structured to support awhispering gallery mode circulating in the optical resonator andoptically coupled to the DFB laser to receive a portion of the laserbeam into the optical resonator in the whispering gallery mode and tofeed laser light in the whispering gallery mode in the optical resonatorback to the DFB laser to stabilize the laser frequency at a frequency ofthe whispering gallery mode and to reduce a linewidth of the DFB laser.This device also includes a resonator tuning mechanism that controls andtunes the frequency of the whispering gallery mode to tune the laserfrequency of the DFB laser via the feedback of the laser light from theoptical resonator to the DFB laser.

These and other aspects and implementations are described in detail inthe drawings, the description and the claims.

BRIEF DESCRIPTION OF DRAWING

FIGS. 1A through 8 show examples and operations of locking a laser and aresonator to each other via an optical interferometer.

FIGS. 9, 10 and 11 show examples and operations of locking a resonatorto a laser by a thermal stabilization using light in a modulationsideband.

FIG. 12 shows a device for locking a laser to a resonator via injectionlocking to achieve a narrow laser linewidth.

DETAILED DESCRIPTION

Optical locking techniques and devices described in this document useoptical resonators with high quality factors to provide narrow resonatorlinewidths. An optical resonator with a high quality factor can belocked relative to a laser frequency of a laser or vice versa. Examplesbelow provide techniques and devices for locking a laser and an opticalresonator to each other. In implementation, a laser is operated toproduce a laser output beam at a laser frequency without modulating thelaser beam. The laser light of the laser output beam is directed into anoptical interferometer which includes a first optical path and a secondoptical path that intersect to produce optical interference betweenlight in the first and second optical paths. An optical resonator iscoupled in the first optical path to filter light in the first opticalpath. Two optical outputs of the optical interferometer are used toproduce an error signal representing a frequency difference between thelaser frequency of the laser and a resonance of the optical resonator.This error signal is then used to control and tune either or both of thelaser and the optical resonator, to lock the laser and the opticalresonator with respect to each other.

FIGS. 1A and 1B show two example devices for locking a laser and aresonator to each other. The device in FIG. 1A locks the resonator tothe laser by tuning the resonator in response to the error signal whilethe device in FIG. 1B locks the laser to the resonator by tuning thelaser in response to the error signal.

In FIGS. 1A and 1B, a laser 101 is used to produce a laser output beamat a laser frequency that is sent into an optical interferometer 110.The optical interferometer 110 in this example is a Mach-Zehnderinterferometer and includes an input beam splitter 113 to split inputlight into a first optical path with a reflector 114 and a secondoptical path with a reflector 116. The first and second optical pathsmeet each other and terminate at an optical combiner 115. In thisexample, the optical combiner 115 can be a partially transmissive andpartially reflective optical device that transmits a part of light fromthe first optical path and reflects a part of light from the secondoptical path to produce a first combined optical output 111 (A). Theoptical combiner 115 also transmits a part of the light from the secondoptical path and reflects a part of the light from the first opticalpath to produce a second combined optical output 112 (B). An opticalresonator 130 is optically coupled in the second optical path to filterlight in the second optical path. A WGM resonator can be used as theresonator 130 and an evanescent couple can be used to couple the WGMresonator to the second optical path. A detection module 120 is providedto detect the first and the second combined optical outputs 111 and 112to produce an error signal representing a frequency difference betweenthe laser frequency of the laser 101 and a resonance of the opticalresonator 130. A feedback control 140 is provided to receive the errorsignal and tunes, one of the laser 101 and the optical resonator 130 tolock the laser 101 and the optical resonator 130 with respect to eachother. In some implementations, both the laser 101 and the resonator 130may be tuned to effectuate the locking.

In FIGS. 1A and 1B, the detection module 120 includes photodetectors,e.g., low-speed photodiodes, to detect the white and dark ports ofinterferometer 110 and the outputs of the photodetectors are subtractedat a differential amplifier 121. The output of the amplifier 121represents the error signal. This output, under perfectly balanced twooptical paths in the interferometer 110, is at a near-zero voltage atthe zero optical detuning between the laser 101 and the resonator 130.The phase of the optical beam in the second optical path with theresonator 130 sharply depends on the frequency of the resonance of theresonator 130 and its bandwidth.

The resonator 130 can be tuned based on various mechanisms. A WGMresonator, for example, can be made from an electro-optic material andcan be tuned by changing the electrical control signal applied to thematerial. In addition, the resonator can be tuned by controlling thetemperature of the resonator or by applying a force or pressure tomechanically squeeze the resonator by using an actuator such as a PZTactuator.

FIG. 2 illustrates signals A, B, and C in FIGS. 1A and 1B. FIG. 3 showsthat the output of the amplifier 121 when operated under a saturatedcondition. The designs in FIGS. 1A and 1B do not require opticalmodulation of the laser carrier from the laser and can be designed to bein a compact package. The feedback can be designed with a high gain toprovide sensitive and effective locking. The locking can be self startedwithin the equilibrium range of the feedback.

The use of the differential signal in FIGS. 1A and 1B limits the lockingrange to be within the linewidth of a resonance of the resonator 130.This operating range may be insufficiently small for certainapplications.

FIGS. 4A-4C and 5 illustrate different implementations of the detectionmodule 120 to provide a broadband locking operation.

FIG. 4A shows that the output of one of the photodetectors is split intotwo signals. FIGS. 4B and 4C illustrate the signals and the detectiondetails for the design in FIG. 4A. One signal is shifted in phase by 90degrees via a phase shifter device 410 and the other signal is sent intoa differentiator 420 that generates a differentiated output of the samephotodetector. The two processed signals are then directed into amultiplier 430 that multiplies the two processed signals. The result isthe error signal which has zero output exactly at the zero frequencydetuning between the laser frequency of the laser 101 and the resonanceof the resonator 130. The signal grows linearly from the zero detuningcondition and has a broad bandwidth which is limited by the bandwidthsof RF components used in the detection module 120.

FIG. 5 shows a device for locking the laser 101 and the resonator 130 toeach other by using a first signal processing unit 410 that shifts aphase of the first combined optical output 111 to produce a firstsignal, a second signal processing unit 420 that performs a timederivative of the second combined optical output 112 to produce a secondsignal. A signal multiplier 430 is then used to multiply the firstsignal and the second signal to produce a multiplied signal. Thedetection module 120 uses the multiplied signal to generate the errorsignal indicating the frequency difference between the laser 101 and theresonator 130.

FIG. 6 further shows an example device that uses both the feedbacktechnique in FIGS. 1A and 1B and the feedback technique in FIGS. 4A-4Cand 5 to lock the laser 101 and the resonator 130 to each other. In thisexample, the device 410 is used to shift a phase of one of the twooptical outputs of the optical interferometer 110 by 90 degrees toproduce a first signal and the device 420 is used to perform a timederivative of the other of the two optical outputs of the opticalinterferometer 110 to produce a second signal. The multiplier device 430multiplies the first signal and the second signal to produce amultiplied signal to generate the first error signal indicating thefrequency difference. In parallel, the differential amplifier 121 isused to produce a differential signal of power levels of the two opticaloutputs of the optical interferometer 110 as a second error signalrepresenting the frequency difference between the laser frequency andthe resonance of the optical resonator. The feedback 140 then uses bothfirst and second the error signals as inputs to another differentialamplifier 610 to produce an output as the error signal to lock the laser101 and the optical resonator 130 with respect to each other.

The above techniques were tested using an experimental setup based on aCaF2 high-Q factor whispering gallery mode resonator filter. A 1551-nmlaser was used in the setup and the laser output is directed into thefiber coupled filter. A Mach-Zahnder interferometer was assembled withfiber patch-cords and Newport direct fiber-couplers. FIG. 7 shows themeasured optical spectrum of the filter and FIG. 8 shows an example ofan error signal.

Photonic filters based on whispering gallery mode resonators (wgmr) mayrequire precise tuning of frequency difference between the carrier andthe resonant frequency of wgmr in various applications. This differencedetermines the center of the filtering function of the photonic filter.For instance eigen-frequency of a single 15-MHz wide CaF2 wgmr filtercan be stabilized within several MHz for certain applications. Thislevel of frequency stabilization may demand a thermal stabilization ofthe resonator within a temperature range of about 7 mK. This can beachieved by using a PID driven thermoelectric cooler (TEC) and passivethermal isolation. Some applications require higher stability. Forexample, a balanced 15M-Hz filter exploits the difference between phasesof two resonant lines. This requires sub-MHz stability of frequencywhich leads to 100 uK level requirement on the thermal stabilization.This level of thermal stabilization may be achieved by using complexmultistage TEC controls techniques, some of which require elaboratethermal design of the filter and relatively bulky packaging.

The following sections describe techniques and devices that exploitnatural thermal nonlinearity of the resonator to control frequencyspacing between the laser carrier frequency of the laser and the mode ofthe resonator. For example, FIG. 9 shows one such device for stabilizinga resonance of an optical resonator with respect. In this device, alaser 901 produces a laser output beam at a laser carrier frequency andan optical resonator filter 920 is placed in an optical path of thelaser output beam to receive light of the laser output beam. An opticalcoupler is provided to couple the optical resonator 920 to the opticalpath to receive the light of the laser output beam and to produce anoptical output. The optical coupler is structured and positionedrelative to the optical resonator 920 to provide, at an optical filtermode frequency different from the laser carrier frequency, opticalcoupling that is not under a critical coupling condition under whichlight coupled into the optical resonator is completely trapped insidethe optical resonator. The device includes an optical modulator 910, inthe optical path of the laser output beam between the laser 901 and theoptical resonator 920, to modulate the laser output beam to produce amodulation sideband at which optical coupling of the light in themodulation sideband by the optical coupler into the optical resonator920 is near the critical coupling condition to thermally stabilize theresonator 920 by optical absorption of light in the modulation sideband.The modulation sideband is different in frequency from the opticalfilter mode frequency. A TEC 930 is in thermal contact with theresonator 920 to control the temperature of the resonator 920 via acontrol by a TEC controller 940. This thermal control provides a roughDC thermal bias to place the resonator 920 near a desired thermalcondition. The modulated laser beam is then used to provide the finethermal stabilization that thermally locks the resonator 920 to thelaser 101.

The laser output beam is modulated at a frequency that is controlled byan RF source or synthesizer 912. The frequency position and theintensity of modulation service sideband can be precisely controlled viacontrolling the RF power and frequency of the synthesizer 912. Thefrequency of the modulation is selected so that the service sidebandcoincides with the auxiliary optical mode which coupling is close butlower than critical coupling condition. The critical coupling conditionfor optical coupling between the optical coupler and the resonator is acondition that the internal resonator loss and the loss of the opticalcoupling via the optical coupler are equal and the light coupled intothe resonator is completely trapped inside the resonator so that thetransmission of the coupled light is zero at the resonance. Under thepresent design, another mode of the resonator 920, a filtering mode thatis used for optical filtering operations, is strongly overcoupled sothat the light in the filtering mode is not trapped inside the resonatorand can transmit through the resonator 920. Therefore, the service andauxiliary modes show very different thermal nonlinearity from thefiltering mode. Practical result of this is that only the auxiliary modepumped with the service sideband affects the optical frequency of theresonator via the thermal nonlinearity while the signal being filteredat the filtering mode does not affect the thermal condition of theresonator.

Absorption of optical power in auxiliary mode results in a reversibleshift of resonator's temperature and a shift in the resonator'srefractive index. This change in temperature shifts the frequencies ofall optical modes near the resonator's rim. The thermal nonlinearityrepresents a natural feedback to fix the optical power in the auxiliarymode. Variations of temperature of the resonator or variations oflaser's frequency lead to optical heating or cooling of the resonator'srim keeping the service sideband at the slope of the auxiliary opticalmode. The feedback gain of this thermal feedback depends on thermalnonlinearity of the resonator, mode volume, thermal conductivity andthermal capacity of the resonator's body.

FIG. 10 shows the frequencies discussed above for the thermalstabilization based on the thermal effect of the service sideband in theresonator. Therefore, the presence of the light in the service sidebandnear the critical coupling condition can locks the filter mode frequencyto the laser carrier frequency without a feedback control circuit. Insome implementations, a feedback control may be provided to furtherenhance the thermal stabilization by the service sideband. For example,an optical detector cab be used to monitor the transmission of the lightfrom the resonator in the service sideband and a feedback control can beused to control either the laser frequency of the laser or thetemperature of the resonator to ensure that the service sideband be atthe slope of the resonance near the critical coupling condition for theservice sideband.

In this technique, the auxiliary optical power can be pumped at anentirely different optical frequency, not affecting the signal beingfiltered at the filter mode frequency. The thermal nonlinearity of thefiltering mode can be kept low. The laser frequency may drift over timeand the thermal locking via the service sideband can maintain thefrequency difference between the laser's carrier and the resonance ofwgmr when the laser drifts. Therefore, a laser of high long-termstability is not required for this locking scheme.

We tested proposed technique with a 200-kHz CaF2 linear wgmr filter, anEO-space modulator and a Koheras 1550-nm laser. To measure the gain ofthermal feedback, the laser frequency was shifted by 100 MHz and therelative shift between laser's frequency and the resonant frequency ofthe mode was measured as shown in FIG. 11. With the service sideband hasan optical power of 120 uW, the feedback time constant was measured tobe around 500 ms and the gain was about 1000. At power higher than 150uW, the feedback of this particular setup became unstable.

The measured stability of the frequency difference was about 100 kHzwhich corresponds to approximately 1 degree of the phase stability in abalanced filter. Further improvements can be achieved with sub-modulatedservice sidebands at frequencies higher than the frequency ofthermo-refractive oscillations. Sub-modulation of this kind suppressesthermo-refractive oscillations, and allows increasing power of servicesideband and as a result increasing gain of feedback.

In operating the above optical resonator filter that is stabilized by aservice sideband, a resonator controlling laser beam at a laser carrierfrequency, that is modulated to carry a modulation sideband, is directedto the optical resonator. The optical coupler is used to couple light atthe modulation sideband into the optical resonator near the criticalcoupling condition to thermally stabilize the resonator by opticalabsorption of light in the modulation sideband. While the resonator isreceiving and is thermally stabilized by the resonator controlling laserbeam, an input optical signal is directed through the optical resonatorto perform optical filtering of the input optical signal by a resonanceof the optical resonator at an optical filter mode frequency that isdifferent from the modulation sideband and the laser carrier frequencyof the resonator controlling laser beam.

Another optical locking is locking a laser to a resonator by injectionlocking. For example, a laser can be locked to a whispering gallery mode(WGM) resonator for line narrowing and stabilization by directing thelaser light out of the laser into the WGM resonator and then feeding thelaser light out of the WGM resonator via direct injection into thelaser. A portion of the light passing through the resonator is reflectedback to the laser to have the laser frequency (wavelength) be locked tothe frequency of the high Q mode of the resonator, and to narrow itsspectral line. If the WGM resonator is stabilized against environmentalperturbations such as temperature variations or vibration, the stabilityof the modal frequency of the resonator is transferred to the laserfrequency or wavelength. The WGM resonator can be made from anelectro-optic material and can be tuned by changing the electricalcontrol signal applied to the material. Because the optical injectionlocking, the laser wavelength or frequency can be tuned with theapplication of a DC voltage applied to the resonator. In addition, byapplying a microwave or RF field to the WGM resonator having a frequencythat matches one or more free spectral range of the resonator, the laserfrequency can be phase, and/or amplitude modulated. Since the modalfrequency of the resonator can be varied by application of temperature,pressure, or in the case of resonators made with electrooptic material,an applied DC potential, the frequency (wavelength) of the laser canalso be tuned. The laser remains locked in frequency (wavelength) to theresonator if the frequency of the laser is modulated through theapplication of a microwave signal to the DC current applied to thelaser. Thus a modulatable, narrow linewidth laser can be obtained. Whenthe WGM resonator is made of an electro-optic material, a microwave orRF field can be applied to the resonator with the appropriate couplingcircuitry to modulate the intensity of the laser, which continues toremain locked to the WGM resonator.

FIG. 12 shows an example device for locking a laser to an opticalresonator. This device includes a distributed feedback (DFB) laser thatis tunable in response to a control signal and produces a laser beam ata laser frequency. An optical resonator is structured to support awhispering gallery mode circulating in the optical resonator andoptically coupled to the DFB laser to receive a portion of the laserbeam into the optical resonator in the whispering gallery mode and tofeed laser light in the whispering gallery mode in the optical resonatorback to the DFB laser to stabilize the laser frequency at a frequency ofthe whispering gallery mode and to reduce a linewidth of the DFB laser.This device also includes a resonator tuning mechanism that controls andtunes the frequency of the whispering gallery mode to tune the laserfrequency of the DFB laser via the feedback of the laser light from theoptical resonator to the DFB laser. The optical coupler in this exampleis a prism coupler for both input and output coupling. Coupling microoptics is provided between the DFB laser and the resonator. The DFBlaser has a high intrinsic Q than other lasers and thus provides abetter control over selection of the WG modes in the dense spectrum of aWGM resonator. The WGM resonator can also be tuned or controlled byapplied strain such as a PZT actuator that squeezes the resonator or bythermal control. As illustrated, the mount on which the laser device isformed is thermally controlled.

While this document contains many specifics, these should not beconstrued as limitations on the scope of an invention or of what may beclaimed, but rather as descriptions of features specific to particularembodiments of the invention. Certain features that are described inthis document in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe excised from the combination, and the claimed combination may bedirected to a subcombination or a variation of a subcombination.

Only a few implementations are disclosed. Variations and enhancements ofthe described implementations and other implementations can be madebased on what is described and illustrated in this document.

1. A device that stabilizes a resonance of an optical resonator withrespect to a laser frequency from a laser, comprising: a laser thatproduces a laser output beam at a laser carrier frequency; an opticalresonator placed in an optical path of the laser output beam to receivelight of the laser output beam; an optical coupler that couples theoptical resonator to the optical path to receive the light of the laseroutput beam and to produce an optical output, the optical couplerstructured and positioned relative to the optical resonator to provide,at an optical filter mode frequency different from the laser carrierfrequency, optical coupling that is not under a critical couplingcondition under which light coupled into the optical resonator iscompletely trapped inside the optical resonator; and an opticalmodulator, in the optical path of the laser output beam between thelaser and the optical resonator, to modulate the laser output beam toproduce a modulation sideband at which optical coupling of the light inthe modulation sideband by the optical coupler into the opticalresonator is near the critical coupling condition to thermally stabilizethe resonator by optical absorption of light in the modulation sideband,wherein the modulation sideband is different in frequency from theoptical filter mode frequency.
 2. The device as in claim 1, comprising:a thermal control that controls a temperature of the resonator to makethe resonator near the critical coupling condition at the modulationsideband.
 3. The device as in claim 1, wherein: the optical resonator isa whispering gallery mode resonator.
 4. The device as in claim 1,comprising: a mechanism to direct an optical signal with spectralcomponents near the optical filter mode frequency to pass through theoptical resonator to optically filter the optical signal by a resonanceof the optical resonator at the optical filter mode frequency.
 5. Amethod for operating an optical resonator filter, comprising: directingto an optical resonator a resonator controlling laser beam at a lasercarrier frequency that is modulated to carry a modulation sideband;operating an optical coupler to couple light at the modulation sidebandinto the optical resonator near a critical coupling condition underwhich light coupled into the optical resonator is completely trappedinside the optical resonator to thermally stabilize the resonator byoptical absorption of light in the modulation sideband; directing aninput optical signal, while the resonator is receiving and is thermallystabilized by the resonator controlling laser beam, through the opticalresonator to perform optical filtering of the input optical signal by aresonance of the optical resonator at an optical filter mode frequencythat is different from the modulation sideband and the laser carrierfrequency of the resonator controlling laser beam.
 6. The method as inclaim 5, comprising: using a thermal control device engaged to theoptical resonator to control a temperature of the optical resonator tomake the optical resonator near the critical coupling condition at themodulation sideband.
 7. The method as in claim 5, wherein: the opticalresonator is a whispering gallery mode resonator.
 8. A device forlocking a laser to an optical resonator, comprising: a distributedfeedback (DFB) laser that is tunable in response to a control signal andproduces a laser beam at a laser frequency; an optical resonatorstructured to support a whispering gallery mode circulating in theoptical resonator, the optical resonator being optically coupled to theDFB laser to receive a portion of the laser beam into the opticalresonator in the whispering gallery mode and to feed laser light in thewhispering gallery mode in the optical resonator back to the DFB laserto stabilize the laser frequency at a frequency of the whisperinggallery mode and to reduce a linewidth of the DFB laser; and a resonatortuning mechanism that controls and tunes the frequency of the whisperinggallery mode to tune the laser frequency of the DFB laser via thefeedback of the laser light from the optical resonator to the DFB laser.9. The device as in claim 8, wherein: the optical resonator comprises anelectro-optic material that changes a refractive index in response to anelectrical signal applied to the optical resonator; and the resonatortuning mechanism applies and controls the electrical signal to tune thelaser frequency of the DFB laser.
 10. The device as in claim 8, wherein:the resonator tuning mechanism controls and tunes a temperature of theoptical resonator to tune the laser frequency of the DFB laser.
 11. Thedevice as in claim 8, wherein: the resonator tuning mechanism appliesand controls a pressure exerted on the optical resonator to tune thelaser frequency of the DFB laser.